1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor and a method of manufacturing the transistor.
2. Description of the Prior Arts
Typically, any conventional npn-type bipolar transistor is provided with an n.sup.+ -type collector which is formed on an n-type silicon substrate by means of epitaxial growth, a p-type base formed on the n.sup.+ collector by means of diffusion, and an n-type emitter formed on the p-type base either by means of diffusion or by applying alloy.
Likewise, any conventional pnp-type bipolar transistor can also be formed in the manner similar to the above.
Each of these bipolar transistors incorporates emitter, base, and collector, which are commonly made of a single semiconductive material, i.e., silicon.
Operation speed of a transistor, which relates to the high-frequency characteristics, depends on the time constants for charging/discharging of emitter-base and base-collector junction capacitances and on the transit time of electrons between emitter and collector.
Maximum oscillation frequency Fmax of a bipolar transistor is expressed as: EQU Fmax=.sqroot.Ft/(8.pi.RbCc) (1)
where
Ft: cutoff frequency
Rb: base resistance
Cc: collector capacitance
Accordingly, when at least one of the collector capacitance and the base resistance is made smaller, the high-frequency characteristics become better. Cutoff frequency Ft has a specific relationship with the transit time of electrons between emitter and collector, which is expressed as: ##EQU1## where
Ce: emitter capacitance
Cp: parastic capacitance
re: emitter differential resistance
Re: emitter resistance
Rc: collector resistance
Consequently, when the emitter capacitance is made smaller, the time needed for electrons to pass through emitter becomes shorter, thus resulting in improved high-frequency characteristics. As a matter of course, when the base width is made narrower the time needed for electrons to pass through the base becomes shorter, thus also resulting in improved high-frequency characteristics. Nevertheless, any of the conventional arts still fails to securely produce such a transistor having less junction capacitance and short base-width in addition to satisfactory ohmic contact by the simple structure based on the simplified manufacturing method as described above. Therefore, it is not possible for the conventional arts to produce transistors featuring satisfactory high-frequency characteristics.
Incidentally, it is known that satisfactory high-frequency characteristics can be securely obtained by a heterojunction bipolar transistor, in which the emitter is formed with a specific semiconductor having an energy band gap greater than that of the base to lower the base resistance. Typical heterojunction bipolar transistors are disclosed in U.S. Pat. No. 2,569,347, U.S. Pat. No. 3,413,533, U.S. Pat. No. 3,780,359, and an article entitled "Heterostructure Bipolar Transistors and Integrated Circuits" written by Herbert Kroemer, Proceedings of The IEEE, Vol. 70, No. 1, Pages 13 through 25, issued in 1982.
The essence of these prior arts is to make up the band of the emitter-base junction by applying a properly selected material so that the band does not substantially act as a barrier to electrons but acts as a large barrier to holes which move from the base to the emitter. As a result, carrier density (hole density) of the base can be significantly increased. This effectively reduces the base resistance, thus realizing an extremely high maximum oscillation frequency Fmax. Typical one of this type of transistors incorporates emitter made of Al.sub.x Ga.sub.1-x As (x&gt;0), and base and collector which are respectively made of GaAs.
However, the junction capacitance still remains large by merely lowering the base resistance. Further, if the base width is narrowed, the electrode cannot easily be drawn out and eventually the external base resistance will increase. Consequently, these heterojunction bipolar transistors cannot satisfactorily improve the high-frequency characteristics.
Japanese Patent Publication No. 55-3829 discloses an improved method for extracting base electrode. In this method, n-type collector layer, p-type base layer and n-type emitter layer are respectively formed on a GaAs substrate by applying liquid-phase epitaxial method. Next, a part of the n-type collector layer is exposed by mesa etching, and then a p-type Al.sub.x Ga.sub.1-x As (x&gt;0) layer is formed on the exposed part by liquid-phase epitaxial method for extracting base electrode. Thereafter, a collector electrode on the collector layer, a base electrode on the p-type Al.sub.x Ga.sub.1-x As layer, and an emitter electrode on the emitter layer are formed.
However, the method proposed by this prior art still has some problems. Concretely, a parastic junction capacitance produced between the emitter and the later-formed base-electrode-extraction p-type Al.sub.x Ga.sub.1-x As layer will cause an increase of the junction capacitance between the emitter and base, and yet, no improvement is done for the junction capacitance between the base and collector. If the emitter were formed before forming the collector on the emitter, the junction capacitance between the base and collector would be reduced, but the junction capacitance between the base and emitter will increase, thus badly affecting the high frequency characteristics.
As is clear from the foregoing explanation, it is quite difficult for any conventional art to securely produce transistors featuring small junction capacitance, narrow base width, and small external base resistance. Consequently, the transistors produced by the conventional arts cannot have fully satisfactory high-frequency characteristics.